Cycle track surface smoothness spec

The nearest the UK has to a standard is LTN 2/08:

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/3808/ltn-2-08.pdf

Surfaces are covered in section 8.8 - but it really only amounts to a discussion. Effectively "Cyclists would like maichine laid tarmac, but if you can only afford loose gravel then that is OK". Cycle tracks will be low in the priority list for maintenace - which is a local authority euphamism for "will not be maintained at all". They certainly won't be gritted or cleared of snow in winter and are very unlikely to be lit.

The only chance of maintenance is when it has deteriorated to the point of virtual non-existance the council may apply for captial funding from some central government sustainable transport fund to build a new track along the line of the old one.

Disclaimer: I'm no expert/professional on the topic but here's some information from Dutch websites:

A document (NL&EN) discussing in how far authorities (should) be liable for damage/injuries to cyclists due to lack of road maintenance:

http://www.fietsberaad.nl/index.cfm?lang=nl&section=kennisbank&kennisbankPage=Onderhoud+en+beheer&mode=detail&repository=Aansprakelijkheid+wegbeheerder+voor+schade+van+fietsers

A document (NL) measuring and comparing the type of cycling infrastructure and maintenance quality among 21  municipalities around the city of Eindhoven:

http://www.fietsberaad.nl/index.cfm?lang=nl&section=kennisbank&kennisbankPage=Onderhoud+en+beheer&mode=detail&repository=Kwaliteitsmetingen+regionaal+fietsnetwerk+SRE

Specifically on page 22, the first table lists the amount of cracks ("scheur"), humps ("hobbel"), dips ("kuil", I don't think this should be interpreted as potholes), and grooves ("kier") in the cycling path surface per 10km. (The final column is the amount of point where paths narrow, per 100km).

A scanned CROW document (NL) from 1995 with section 3 containing guidelines for when maintenance is required. I haven't read it in detail and at a glance couldn't find many exact numbers, but it shows some images with damaged surfaces that are considered in need of repair.

http://www.fietsberaad.nl//index.cfm?lang=nl&section=kennisbank&kennisbankPage=Onderhoud+en+beheer&mode=detail&repository=Rationeel+beheer+van+fietspaden#Comments

Finally, my personal experience is that local government typically acts quicky on complaints by cyclists. On my old commute route in Utrecht, they started road works, and a (not so) temporary cycle path was laid down with concrete surface elements. People complained about the (slight) non-levelness between the elements, so it was replaced by machine-laid tarmac. Later the transition from neighbouring surface to tarmac was improved, because it was a bit "bumpy".